docs: def correl_model(self):
          """
          Return a correlation object. See :mod:`openquake.hazardlib.correlation`
          for more info.
          """
          correl_name = self.ground_motion_correlation_model
          if correl_name is None:  # no correlation model
              return
          correl_model_cls = getattr(
              correlation, '%sCorrelationModel' % correl_name)
          return correl_model_cls(**self.ground_motion_correlation_params)